This invention pertains to antenna switches and, more particularly, to an antenna switch that automatically switches from a first to a second antenna when the second antenna is connected to the switch.
FIG. 1 is a schematic diagram of a prior art, four terminal, remotely controlled, single-pole double-throw (SPDT) antenna switch 100. Radio terminal R is connected to the RF input/output terminal of a non-illustrated radio transceiver. Antenna terminal A1 is connected to a first or local antenna 101 while antenna terminal A2 is connected to a second or remote antenna 118. The DC voltage at a separate control terminal C determines whether radio terminal R is coupled to antenna terminal A1 or A2.
When the control voltage source 102 and, consequently, the voltage at control terminal C are a ground potential, a DC current flows from a source of positive voltage V+ through resistor 104, RF choke 106, diodes 108 and 110, and RF choke 112. This current causes diodes 108 and 110 to switch to the conducting state. When diode 110 is conducting, capacitor 114 and inductor 116 form a parallel tuned circuit which is anti-resonant at the operating frequency. This anti-resonant parallel tuned circuit decouples radio terminal R from antenna terminal A1 by inserting a high impedance in the path between these two terminals. Since diode 108 is in the conducting state when control terminal C is grounded, radio terminal R is coupled to antenna terminal A2 through diode 108 and capacitors 120 and 122.
When the voltage at control voltage source 102 and control terminal C are at V+, no DC current flows through diodes 108 and 110, and the diodes switch to the open-circuit state. Since diode 108 is in the open-circuit state, radio terminal R is decoupled from antenna terminal A2. When diode 110 is in the open-circuit state, capacitor 114 and inductor 116 no longer form an anti-resonant parallel tuned circuit. Consequently, radio terminal R is coupled to antenna terminal A1 through inductor 116 and capacitors 120 and 124.
FIG. 2 is a schematic diagram of a prior art, four terminal, transmit/receive ("T/R") switch. Transmit terminal TX is connected to the output of a non-illustrated transmitter, and receive terminal RX is coupled to the input of a non-illustrated receiver. When the voltage at input control terminal C is sufficiently above ground potential, current flows through RF choke 202, diode 204, quarter wavelength transmission line 206 and diode 208, thereby switching diodes 204 and 208 to the conducting state. When diode 208 is conducting, terminal 206A of transmission line 206 is shorted to ground. This causes the impedance at the other terminal 206B to go to infinity, thereby decoupling antenna terminal A from receive terminal RX. Since diode 204 is conducting, transmit terminal TX is coupled to terminal A and antenna 210 through capacitor 212 and diode 204.
When the voltage at control terminal C is near ground potential, no current flows through diodes 204 and 208. Consequently, diodes 204 and 208 are switched to the open-circuit state. Thus, antenna terminal A is coupled through transmission line 206 and capacitor 214 to receive terminal RX. Since diode 204 is in the open-circuit state, transmit terminal TX is decoupled from antenna terminal A.
The switches illustrated in Figs. 1 and 2 are both SPDT RF type switches. In addition to the three terminals of a conventional mechanical SPDT switch, both prior art switches require an additional control terminal C. Although the invention described below also provides SPDT RF switching, it has a distinct advantage in that the separate control terminal C has been eliminated. Thus, the invention only requires three terminals. The invention has an additional advantage in that switching from a first antenna to a second occurs automatically, simply by connecting the second antenna to the switch.